Conventionally, a quantitative school of fish detector for performing a quantitative measurement of fish for the purpose of, for example, a fishery resources research has been known. This kind of quantitative school of fish detector obtains a ratio of a sound pressure level of an ultrasonic wave discharged from a transducer and a sound pressure level of an echo that is reflected by a fish, that is, a reflection strength TS (target strength), and calculates a length of the fish from the reflection strength TS by using a relation of the reflection strength TS and a fish body length L.
In a quantitative measurement of fish, a single fish needs to be detected discriminately from a school of fish. For example, JP05-196733A discloses a single fish determination circuit using ultrasonic wave transmission pulses with two kinds of pulse widths, respectively. That is, according to the description in JP05-196733A, two kinds of ultrasonic transmission pulses with long and short pulse widths, respectively, are sequentially transmitted and, after reception signals of these pulses are formed into a rectangular shape, it is determined whether the reception signals are from a single fish based on a ratio of the rectangular widths of the corresponding reception signals. For the determination, phenomena is utilized in which in a case of the single fish, the ratio of the rectangular widths of the two long and short reception signals remains within a predetermined range known in advance, whereas, in a case of other cases including a school of fish, because the rectangular widths of the reception signals become longer as a length of the school of fish increases in the advancing direction of the ultrasonic wave, and the ratio is different from the case of the single fish case.
In JP05-196733A, because the two kinds of ultrasonic transmission pulses with the long and short pulse widths, respectively, are used, a configuration of the circuit for the above ultrasonic transmission pulses is needed, and the transmission of the ultrasonic transmission pulses with the long and short pulse widths, that is the two wave transmissions, correspond to a single detection operation, therefore, detection efficiency correspondingly degrades.
Meanwhile, a method of detecting a single fish from reception signals of a single frequency ultrasonic wave transmission signals by extracting rising and falling components thereof is also known. This detecting method is, for example, a method of sampling the reception signals at a high sampling speed and detecting the rising and falling components from the sampled signals, and a method of obtaining an average value or extracting a maximum value of the reception signals for each sampling period at a sampling speed slower than the above high-speed sampling, and detecting the rising and falling components by using the signals. However, with this method, an amount of data that is sampled at the high sample speed becomes extremely large and interferes with high-speed processing. Further, in the case where the rising and falling components are to be detected by using data processed through the average value processing and the maximum value processing, there has been a certain limit to an accuracy in detecting the single fish.